1. Field of the Invention
This invention relates to fluid compositions. More particularly, this invention relates to fluid compositions comprising a colloidal dispersion of an aromatic ether represented by the formula ##STR2## wherein R.sup.1, R.sup.2, and R.sup.3 independently are phenyl biphenyl, and terphenyl and n is an integer of from zero (0) to 5, and an oxidation inhibiting amount of an alkali metal salt of oxalic acid.
The fluid compositions of this invention are useful in a number of applications requiring fluids resistant to oxidative and thermal degradation under use conditions of high stress (such as elevated temperatures of 316.degree. C. (600.degree. F.) and higher. For example, the fluid compositions are useful as atomic reactor coolants, diffusion pump fluids, damping fluids, force transmission fluids (hydraulic fluids), heat transfer fluids, and synthetic lubricants, particularly as jet engine lubricants.
2. Description of the Prior Art
Aromatic ethers, particularly polyphenyl ethers (wherein each of R.sup.1, R.sup.2, and R.sup.3 is phenyl) and their use as functional fluid compositions are well known in the art. They are oxidatively stable to about 275.degree. C. (527.degree. F.) and resist pyrolysis to about 445.degree. C. (833.degree. F.). However, at temperatures above their stability limits, the aromatic ethers tend to develop sludge and thicken to a degree which adversely affects their high temperature performance. Therefore, a variety of additives have been proposed and disclosed in the prior art to stabilize the aromatic ethers against oxidative degradation and thereby extend their operation range.
The susceptibility of the aromatic ethers to some degree of oxidative stabilization by a variety of metals (or their oxides and carboxylates) was reported for polyphenyl ethers by Ravner et al, J. Chem. Eng. Data, 8, 591-596 (1963).
U.S. Pat. No. 3,245,907 discloses the stabilization of polyphenyl ethers against oxidative degradation by the incorporation therein of an organotin compound represented by the formula EQU R.sub.m --Sn--X.sub.4-m
and EQU R.sub.3 --Sn--Sn--R.sub.3
wherein R is an alkyl, aryl, aralkyl, aryloxyaryl, biaryl, thienyl, and pyridyl group; X is R or a halogen, and m is an integer (whole number) of from 1 to 4.
In U.S. Pat. No. 3,290,247, polyphenyl ether compositions having improved antioxidant properties are disclosed and claimed. The oxidative stabilization of the polyphenyl ethers is achieved by incorporating therein organotin compounds represented by the formula EQU R.sub.n Sn(XR').sub.m (I)
wherein n is an integer of from 2 to 3, m is an integer of from 1 to 2, and the sum of m+n is 4; R is an alkyl group (referred to in the reference as a radical) of from 1 to 12 carbon atoms, benzenoid hydrocarbon groups which are free of olefinic and acetylenic unsaturation and contains from 6 to 12 carbon atoms, and aryloxyaryl groups of from 12 to 24 carbon atoms, and such groups having halogen substituted at the benzenoid nucleus; R' is R, paraffinic and haloparaffinic acyl groups of from 2 to 12 carbon atoms, the group ##STR3## wherein Z is a necessary member to complete a saturated heterocyclic group of from 6 to 10 members, the groups --SnR.sub.' and --arylene--O--SnR.sub.3 ; and X is a chalcogen element having an atomic weight of less than 33, and those represented by the formula ##STR4## wherein Y is an arylene group of from 6 to 12 carbon atoms, an arylenealkylenearylene group and alkylenearylenealkylene group having from 1 to 4 carbon atoms in the alkylene group and from 6 to 12 carbon atoms in the arylene radical, R and X are as previously defined, and y is a number of from 10 to 1000 (to denote the degree of polymerization).
U.S. Pat. No. 3,492,229 discloses, inter alia, aromatic ether compositions which exhibit improved oxidation resistance. Such compositions are provided by incorporation of organic salts of alkali metals, antimony, bismuth, and lanthanum into the aromatic ether basestock. Such compositions reportedly are useful as jet engine lubricants, heat transfer fluids and hydraulic fluids.
Although these prior art aromatic ether compositions generally exhibit increased oxidation stability over aromatic ethers themselves, they, in general, are limited in their application usefulness as functional fluids and engine lubricants under high stress conditions extant in present-day high stress engines--jet engines, for example--as well as engines currently under development for the next generation of jet engines. For example, as the speed and altitude of operation of jet engine-containing vehicles increases, lubrication problems also have increased because of increased operating temperatures and higher bearing pressures resulting from the increased thrust needed to obtain and maintain high speeds and altitudes. And as the service conditions encountered become increasingly severe, the useful life of the functional fluid is shortened, primarily due to their deficiency in oxidative stability above about 275.degree. C. (550.degree. F.). In general, as operating requirements of a jet engine are increased, engine temperatures increase and oil temperatures in the range of 316.degree. C. (600.degree. F.) and higher are encountered. Accordingly, research efforts are continually being made to define new or improved fluid compositions, particularly aromatic ether compositions, and most particularly polyphenyl ether compositions, which exhibit increased oxidative stability and concomitantly are suitable for use as functional fluids under use conditions of high stress. The discovery of the fluid compositions of the instant invention, therefore, is believed to be a decided advance in the functional fluid art.